When Does Shallow Water Become
a Problem ?
A HydroComp Technical Report
Report 124
During sea trial analyses,
users of NavCad and PropExpert have encountered situations where
the drag and power were much greater than expected. In some of
these instances, the shallow water effects may be the cause of
this added drag and power. This report is written to help determine
when shallow water becomes a problem.
Overview
Boats and
ships can be easily affected by shallow water. Basically, we are
witnessing a "Bernoulli" effect, where the water velocity around
the hull must be accelerated due to the restriction of shallow
water. This increase in velocity results in a higher drag comparable
to this higher "effective" speed. In addition to this, the accelerating
water can pull the vessel down (known as sinkage or squat), causing
the ship the act as if it had a larger displacement. So when does
shallow water cause a measurable loss of speed?
Shallow
Water
We describe
the parameters of shallow water through the "depth Froude number",
which is equal to V / (gH)^{0.5}. The parameter V is the
ship speed, g the gravitational constant and H the water depth.
In units of knots and feet, F_{NH} = 0.297 V_{KT}
/ (H_{FT})^{0.5}. In knots and meters, F_{NH}
= 0.164 V_{KT} / (H_{M})^{0.5}.
One widelyused
analysis of speed loss in shallow water is the Schlichting method.
It can give us a general sense of the potential speed loss. He
found that there is typically no measurable speed loss as long
as F_{NH} is less than about 0.4. As F_{NH} increases,
however, the speed loss begins to take effect:
F_{NH} 
0.00.4 
0.6 
0.8 
1.0 
Speed
loss 
no
loss 
1%
loss 
4%
loss 
14% loss 
(NOTE: When F_{NH}
is greater than 1.0, we typically see the speed loss quickly reducing
to where we can even get a beneficial effect at high speeds, such
as at planing speeds.)
We can turn this into
a table of "critical depth" for each F_{NH}:
Speed 
Minimum
depth for 
no
speed loss 
1%
speed loss 
4%
speed loss 
14% speed
loss 
5 kts 
14+
ft 
6
ft 
 
 
10
kts 
55+
ft 
24
ft 
14
ft 
9
ft 
15
kts 
124+
ft 
55
ft 
31
ft 
20
ft 
20
kts 
220+
ft 
98
ft 
55
ft 
35
ft 
This tells us that at "displacement"
speeds, the vessel will slow down as water depth gets shallower.
It also shows that "shallow" water need not be all that
shallow, particularly at higher speeds. Remember  these are typical
values only and each ship will be somewhat different.
Examples
 A fishing boat normally
can run at 10 knots in deep water. If the water depth is 14
feet, then we can expect a speed loss of about 4%. That means
that its speed is reduced by about onehalf knot.
 A supply boat is expected
to run 15 knots in deep water. However, during sea trials in
20 feet deep water it did not make this speed. The 14 % typical
speed loss means that we would expect a bit less than 13 knots
on trial.
 A river work boat is
trying to run at 10 knots in 9 ft of water. It will lose almost
a knot and a half.
